System and method of generating seismic waves in the earth



March 3, 1953 2 SHEETS-SHEET l Filed Jan. 11, 1947 RECORDER FYING ANDAPPARATUS wlz FIG.

m M a w A 7 H. w MZ D m m x/ March 3, 1953 J. E. HAWKINS ETAL 2,630,188

SYSTEM AND METHOD OF GENERATING SEISMIC WAVES .IN THE EARTH Filed Jan.11, 1947 .2 SHEETS-SHEET 2 25 28 7 G- 4 27 x a Series Connected ToIgnition Elements Of Other Charges I70 37 Outer Charges 0009 Delayed n nOuter Charges d -ii INVENTOILD 39 James/5 Hawkins Deioyed 39acenfefichqrge BY Will/am E Pugh Delayed Mw Affomev Patented Mar. 3, 1953UNITED STATES PATENT OFFICE SYSTEM AND'METHOD OF GENERATING -SEIS1WICWAVES IN EARTH v. PS' 1 1 7- The present invention relates to the art ofseismic prospecting and more particularly to an improved systemandmethod of generating seismic waves in the earth.

Fundamentally, the art. of seismic surveying is based upon thegeneration of sound or seismic waves in the earths crust and detecting,recording and interpreting the .waves which ,are reflectedand refractedback to the earthssurface from buried strata interfacesand the like.Present practice in the generation of seismic waves isthat of detonatingdynamite or,other explosives, as commercially obtained in package form,or as placed in. a suitable container, metal or otherwise, to facilitateloading, in;a shothole usually drilled below the weathering ,or lowvelocity layer of theearths crust. Detonation of the explosive chargebelow the weathering layer is usually necessary inorder toavoidgenerating excessive amounts of undesired surface or near surfaceseismic. wave. energy which travels horizontally or nearly .so-to thepoints of wave detection. These undesired .waves frequently havefrequencies approximating the frequencies of the desired reflectedandrefractedwaves and hence defy filtering prior to recording. .Theyhave the effect of .so confusingor obscuring the records of thedesiredrefiected wavesas torender record interpretationdifiicult orimpossible. In many areas, the interferingwave energy is of such largemagnitude as compared .with the magnitude of the reflected wave. energythat suc- CElSflll reflection prospecting is either very difficult andexpensive or impossible. even when shot detonation below the weatheringlayer is resorted to to reduce the undesired .waveenergy. Moreover, eventhough this method of .shooting .is often successful in reducing. theinterferingwave energy to a tolerable value, it is somewhat.expensive inthat the shot holes must be drilled to considerable depths. In extremelyunfavorable areas, undesirable energy cannot be eliminated even when theshot holes are drilled below the weathering and further, heavy andexpensive explosive charges must be used in order to obtain reflectedwave energy. in sufficient magnitude to permit accurate interpretationof the resulting record.

It is an object of the present invention, therefore, to provide animproved system and method for so generating seismic waves in the. earththat more effective utilization of explosives is obtained,

thereby to conserve explosives and to eliminate the necessity ofdrilling shot holes through the weathering layer except in unusuallydifficult a downward direction,thereby to dec'reasethe;

amount ofundesired horizontal wave energy produced and to increase theamount of wave .energy propagated downwardly toward the re- .fiectinginterfaces.

.It .is still another object of the invention to at least partiallysuppress certain undesiredinterfering (horizontal) waves of .a selectedand particularly objectionable frequency.

.The invention,.both as to its organization and method of. operation,togetherwith other objects and advantagesthereof will best be understoodbyreference tothe following specification taken in connection with theaccompanying drawings in which Fig. 1. illustrates a seismic surveyingsystemembodying a system of generating seismic waves characterizedby thefeatures of the present invention;

Fig. 2 illustrates the shot hole pattern ofthe seismic wave generatingsystem embodied in the seismic surveying system shown in Fig. 1;

=-Fig. 3 is a view partially in section illustrating the details of oneof the cavitated charges preferably used in each of the-shot holes shownin -Figs. 2 and 4;

Fig. l is a-view smilar to Fig.1, but illustrating electrical equipmentfor detonation of the center charge and the outer charges at the sameor-different times; and

Figs. -5 and6 graphically illustrate the degrees of directionalizationobtained when the center and outer charges are at the same and differentlevelsin the earths crust and are fired simultaneously and with delaytimes between the center and outer charges.

Referring now-to the drawings and more particularly to Fig. lthereof,the present improved fully below, detonationof the explosive charge 'orcharges employed in the present improved wave generating system iseffected under the control of a conventional electric impulsetypedetonator Hi whichis electrically connected by means of a cable ltd withone of the signal translating channels of the apparatus l2 for thepurpose of transmitting the time-break signal to one of the recordingelements of the recorder i3. This detonatortogetherwiththe signaldetectors :H, the amplifying and filtering apparatuslZ-andthe recorderi3-may be of any desired commercial construction.

In general, the mode of operation of the system as thus far describedwill be readily understood by those skilled in the art. Briefly,however, seismic signals generated in the manner more fully pointed outbelow at the wave source or shot point it] distant from the array ofdetectors H, and as transmitted through the earth both directly andthrough reflection and refraction from buried strata interfaces and thelike, are picked up by the detectors I I, converted into correspondingelectrical signals through the operation of these detectors, amplifiedthrough the respective amplifying channels of the apparatus l2 andimpressed upon the respective associated driving coils of thegalvanometer elements embodied in the recorder [3 for recording. Also inaccordance with conventional practice the time-break signal, as producedsimultaneously with generation of the seismic waves, is transmitted overthe cable Ma and through at least a part of one of the channels of theapparatus 12 to one of the recording elements of the recorder l3 forrecording.

As pointed out above, the conventional method of generating seismicwaves required in practicing the above described method of seismicsurveying is that of drilling a shot hole through the weathering or lowvelocity layer 9 of the earth's crust, depositing an explosive chargeusually in the form of packaged dynamite at the bottom of the shot holeand detonating the charge. This system of seismic wave generation isopen to the several disadvantages briefly outlined above. In accordancewith the present invention these disadvantages are at least in partobviated through use of the present improved seismic signal generatingsystem to directionalize the seismic wave propagation and to at least inpart suppress interfering wave energy. In one embodiment of theinvention, these ends are accomplished by depositing a plurality ofexplosive charges ii in a plurality of shot holes 15 and It to form anexplosive charge pattern of approximately conical configuration whichsimulates a large cavitated charge and has the effect of producing massdirectionalization of the generated seismic waves in the mannerexplained below with reference to Figs. 5 and 6 of the drawings when theexplosive charges 11 are detonated. More specifically and as best shownin Fig. 2 of the drawings, the shot hole I5 is drilled at the center ofa closed loop, preferably in the form of a circle 16a, and the shotholes [5 are drilled at equally spaced points around the circumferenceof this circle. In order to obtain the desired directionalization of theexplosive energy, the center shot hole It: is drilled to a depth A notgreater than the depth B to which the surrounding shot holes [6 aredrilled and usually less. For example, these depths are preferably soproportioned that the cone angle E is of the order of 90 if all of thecharges ll are to be simultaneously detonated. The detonating caps ofthe respective explosive charges II are commonly connected by means oftwo-conductor cables Mb to the current impulse generating circuit of thedetonator [4 (which may be of the schematic form shown in Fig. 4 of thedrawings) so that when this detonator is operated, the charges areexploded either simultaneously or in sequence.

As explained more fully below with reference to Figs. 5 and 6 of thedrawings, with the explosive charges ll arranged in a conical pattern tosimulate a large cavitated charge, the well 4 known Monroe effect isobtained when these charges are either simultaneously detonated ordetonated in a predetermined order. Specifically, the mass concavity ofthe shaped arrangement of explosive charges in the several shot holes inpart causes the seismic energy to be directionalized in bulk in thesense that the wave energies produced at the individual shot holes arefocused toward the central portion of the group pattern. In thearrangement illustrated, wherein the apex of the cone is nearest theearths surface 8, the mass directionalization of the generated seismicenergy is in a downward direction. Due to the describeddirectionalization of the generated waves, the available explosiveenergy per unit of explosive material is much more effectively utilizedfor the reason that the increase in seismic energy propagation downwardis realized at the expense of seismic energy propagation alonghorizontal or near-horizontal paths. As explained above, however,reduction in the horizontal seismic energy propagation is highlydesirable since it is this energy which gives rise to interfering wavesat the detectors H having the effect of obscuring or completelyobliterating the reflected and refracted waves which must be recorded topermit accurate interpretation of the attitude of the subsurfacestructure. Further, the increased effectiveness of each available unitof explosive material in producing wave propagation in a downwarddirection has the effect of further reducing the amount of explosiverequired to produce the desired amount of reflected and refractedenergy. Thus while several charges are employed, the aggregate of theexplosive material in the several charges may be appreciably less thanthat in a single charge of suflicient size to produce the same amount ofseismic energy propagation in a downward direction. This furthercontributes to the desired reduction in undesired wave propagation indirections extending horizontally away from the shot holes.

A further advantage of the described syste resides in the fact thatshallower shot holes may be used to receive the explosive charges. Thusin the arrangement illustrated in full lines in Fig. 1 of the drawingthe shot holes I5 and I6 are not drilled through the weathering layer 9.This is possible by virtue of the increased effectiveness of theexplosive charges in producing seismic energy propagation in a downwarddirection and reducing the seismic energy propagation in the horizontaldirections. In most areas where favorable conditions prevail, shot holesl5 and i6 drilled to very shallow depths in the weathering layer 9should sufflce to produce entirely acceptable records. However, incertain very difficult areas of deep weathering (exceeding one hundredfeet, for example) and in areas where there is a relatively large amountof interference energy, it may be necessary to drill the holes l5 and 16to depths C and D, respectively, extending below the bottom 9a of thelayer 9. In such case, also, a differential between the depth C of thecenter hole I5 and the depth D of the surrounding holes 16 may bedesired in order to realize the described mass concavity efiect whichresults in directionalized propagation of the seismic energy in adownward direction when the charges in the several shot holes aredetonated.

As indicated above, with the described system a reduction in theinterfering or horizontally propagated energy is inherently obtained dueto racemes the directionalization of the;generated seismic energy in adownward direction. In the usual case, the desired reflected andrefracted waves fall within a restricted frequency range. This frequencyrange may varyfrom area to area de- S pending upon the character .of thesurfaceand subsurface, but usually falls in a range of from 30 to'lOOcycles per second. It is desirable, therefore, particularly inunfavorable areas where difficulty is experienced in obtainingappreciable.

wave reflection and refraction,'to .suppress or minimize the interferinghorizontally. propagated waves of the same frequency since, due to thefrequency similarity of the vdesiredand undesiredenergy, filteringwithin the apparatus detonation of the outer charges are cancelled bywaves of the same frequency produced by detonation of the center charge.If the charges are to be detonated simultaneously, this may beaccomplished by making the radius of the circle I 6 a finite fraction ormultiple of the wave length of the particular frequency at which thedesired energy is to be received. For example, if test shots indicatethat the. interfering energy occurs at .a frequency of twenty cycles persecond and also that the velocity of wave propagation through theweathering or low velocity layer 9 is two thousand feet per second, thewave length of the twenty cycle energy obviously is one hundred feet. Insuch a case, by arranging the surrounding shot holes I6 symmetricallyabout the center of the shot hole It on a circle having a radius offifty feet, the twenty cycle wave energy generated in the center shothole I will be approximately l80 out of phase with the twenty cycle waveenergy generated in the surrounding holes I6 in a horizontal plane,resulting in cancellation of a considerable amount of twenty cycle waveenergy in directions extending lat- As a further example,

erally from the shot holes. if the shot holes are drilled below theweathering or low velocit layer 9 in the manner illustrated in dashlines in Fig. 1 of the drawings and tests indicated that suppression ofan interfering fifty cycle wave energy is desirable and that thevelocity of wave propagation in the strata below the weathering layer 3is eight thousand feet per second, waves of the frequency indicated willhave a wave length of one hundred and sixty feet. Therefore, to obtainsuppression of the interfering energy of this Wave length in ahorizontal direction, the outer shot holes It should be symmetricallyspaced about the inner shot hole I5 on the circumference of a circlehaving a radius of eighty feet. Thus by a proper choice of the length ofradius of the. circle on which the outer shot holes I6 are" located,propagation of undesired interfering wave energy may be furtherminimized.

As will be evident from the above explanation, partial suppress on ofundesired hori ontal Wave energy propagation is obtained when theone-half length wave length spacing of the outer charges Il from thecenter charge I1 is used and the charges are simultaneously detonated.In accordance with still another feature of the present invention,suppression of undesired energy propagation laterally from the shotholes is also obtained by employing the .detonating'or firing ystemillus-t edownward. direction.

trated in Fig.- 4 of the drawings to; detonate the center charge Ilbinthe center shot-hole I511 -at-a different time than-the outer-chargesIla .intheshot hOles Ifia. .In brief, the firing-system shown in Fig.4ofthe drawings comprises a-cur- .rent source 26 for energizing theignition elements of-the charges Ila and Ilb, an on-ofi switch 2land acommutating device for energizing the ignition element of the centercharge .I'lbeither'before, after or at the same time as the outercharges Ila are energized. This =commutating device is-adapted to be.driven about its axis of rotation. 3| by means of anyconventionalconstant speed motor and is comprised of =-a slip-ring Z-Bhaing acommutating segment 28a interposed between insulating rings 29 and3 D. The slip ring 28 projects radially beyond the ends of the rings .29and 30 I and the projecting end portion i constantly engaged at itsinner periphery by a carbon':brush-32. Two additionalbrushes-33 and -34are'provided which are spring urged to engage the periphery of theinsulating ring30 and to be successively engaged by the commutatingsegment 28a of the ring 23 during each revolution of the commutatorassembly. With-this construction and with the commutating device -25 inoperation, circuits are successively completed for energizing theignition element of: the

outercharges Ila and then the ignition element "of the center charge Ilbin response to-closure of the switch 27. Thusyby observation of-thecommutating device (which travels at observable speed in acounterclockwise direction), the switch 27 may be closed while thecommutating segment 28 is traveling away from the brush 34 and towardthe brush 33. With this switch closed, a circuit including the brush 32,the commutating ring 28, the-segment 28a and the brush 33,- is firstcompleted for energizing the ignition elements of the four outer chargesIla in series, thereby to effect simultaneous detonation of thesecharges, simultaneously a time break signal is transmitted over thecircuit leads 25a to the distant amplifying and filtering apparatus. Apredetermined time interval thereafter (this intervalis determined bythe speed of the commutating device 25 and the spacing between thebrushes 33 and-34), the commutating segment 28a engages the brush 34 toenergize the ignition elements of the charge I lb in series with thecurrent limiting resistor 35 (which is equal in reas desiredby varyingthe spacing between the brushes 33 and 34. Preferably, therefore, the

-brush.34 is arranged for adjustment around the .ring 30 to any desiredposition on either side of the brush 33 or to the same position as thelatter brush, so that by appropriate adjustment of the brush 34, thecenter charge Ilb may be det- -onated any desired determined timeinterval before'or .after detonation of the outer charges Ila orsimultaneously with these charges.

Detonat ng the center and outer charges in sequence with varying degreesof delay between the detonation of center charge Ilb and thesimultaneous detonation of the outer charges Ilahas the effect ofchanging the character of the directionalization 'of the explosiveenergy in a Specifically and as .bBSt

shown in Fig. of the drawings, when any given charge is detonated, awave front is generated which travels symmetrically outward in alldirections away from the shot point at the same speed. In reference tothe charge I'Ib, this wave front in traveling away from the charge isrepresented by a series of expanding circles 36. Two similar series ofexpanding circles 3'! and 38 represent travel of the wave fronts awayfrom the left and right shot points, respectively, when the charges I'Iaat these points are detonated.

It is the positions of the meeting points of the wave fronts producedthrough detonation of the center and outer charges relative to thelocations of these charges which determine the degree ofdirectionalization of the propagated energy. Thus with the center chargeI'Ib disposed in the earths crust at a lesser depth than the outercharges Ila, the wave fronts developed upon simultaneous detonation ofthe center and outer charges will meet at points between the center andouter charges and equidistant therefrom. In other words, the wave frontsof the center and outer charges meet at some point along each of thelines 39a and 3% which are the locus lines of the points of intersectionof the wave front circles 36 individual to the center charge II'b withthe wave front circles 31 and 38 of the same diameter individual to theleft and right outer charges. The angle of convergence between the lines39a and 391) thus may be used as an index of the degree ofdirectionalization of the generated explosive energy in a downwarddirection.

Considering the circle diagram shown in Fig. 5 further, it will beunderstood that if detonation of the center charge IIb is delayed for aninterval after the outer charges Ila are fired, the wave frontstraveling away from the outer shot points Ila will be further removedfrom their respective source points than the wave front traveling awayfrom the center shot point I'll) at the meeting points between thecenter and outer wave fronts. Thus for one delay interval, the wavefronts generated upon detonation of the outer charges IIa will meet thewave front generated upon detonation of the center charge ilb atcorresponding points along the locus lines 49a and 431). If a longerdelay interval is used, wave front collision will occur at correspondingpoints along the locus lines Ma and Mb. On the other hand, if the centercharge I'Ib is detonated ahead of the outer charge IIa, the wave fronttraveling away from the center shot point will be further removed fromthis point than the wave fronts traveling away from the respective outershot points at the points of collision therebetween. Depending upon themagnitude of this delay interval, wave front meetings may occur atcorresponding points along corresponding locus lines Ha-42b, 43a-43b,etc.

From the above explanation with reference to Fig. 5 of the drawings itwill be clearly apparent that the degree of directionalization ofexplosive energy propagation in the desired downward direction may bechanged as desired by changing the time relationship between the instantof detonation of the center charge Ilb and the instant of detonation ofthe outer charges I'I'a. In general, best results are obtained when thecenter and outer charges are detonated to produce wave front meeting atthe same points along loci lines having an angle of from to 60convergence. Another factor or parameter which determines the degree ofdirectionalization of explosive energy propagation is the relative depthof the centerand outer charges. This will be clearly evident from aconsideration of Fig. 6 of the drawings wherein the'center charge I'Ibis shown as being at the same depth as the outer charge i'la. From acomparison of the lines 39a and 39b representing the loci of the wavefront meeting upon simultaneous detonation of the center and outercharges having the same depth as shown in Fig. 6 of the drawings withthe corresponding lines 39a and 39b shown in Fig. 5 of the drawings torepresent wave front meeting upon simultaneous locus lines of Figs. 5and 6 respectively, repre.

senting delayed center charge detonation and delayed outer chargedetonation. It will thus be apparent that the relative depths of thecenter and outer charges is a second factor or parameter which may bevaried to govern the degree of explosive energy directionalization whichmay be obtained upon detonation of the several charges. At appreciabledistances away from the shot holes, this factor has no observable effectupon the cancellation of undesired waves in a horizontal direction, i.e., laterally of the shot holes.

As indicated above, however, the time relationship between the firing ofthe center charge and the several outer charges has a marked effect uponthe suppression of undesired energy propagation in a horizontaldirection away from the shot holes for the reason that this relationshipdetermines the phase relationship between waves radiating from theseveral shot points following the initial impulse or transient wave.Thus if the waves radiating from the center shot hole I5a are in phasewith the waves radiating from the outer shot holes IGa, undesired energyis additive. On the other hand and as explained above, if the wavesradiating from the center hole Ida are out of phase with those radiatingfrom outer shot holes I6a, the waves tend to cancel and energysuppression is obtained. This latter effect may be realized in themanner explained above by making the radius of the circle on which theouter shot holes Ilia are located equal to one half the wave length ofthe waves representing the undesired energy. The desired out of phaserelationship may also be obtained by properly delaying detonation of thecenter charge Ilb for a predetermined interval after the outer chargesI'Ia are detonated. In general, maximum undesired energy propagation ina horizontal direction is obtained when the following relationshipprevails:

l DT= L where:

DT=the delay interval in milliseconds separating detonation of the outercharges Ila and later detonation of the center charge IIb;

R=Radius of the circle on which the outer shot holes IGa are located;

V=Velocity of wave propagation in the horizontal direction in feet permillisecond;

\=Wave length of the undesired wave energy.

From this relationship, it will be observed that any desired radius R ofthe circle on which the aesoussouter shot holes lea are located may-beused,

providing an" appropriate adjustment is made ters, i. -e., the factorsDT and R'governundesired energy suppression. Further, the two factors DIand the relative depths of the center and outer explosive charges may bevariedtoobtain the desired degree of directionalization-of explosiveenergy propagation in adownward direction. Permissible variations inthese three parameters permit optimumresults in the form of maximumsuppression of undesired waves andmaximum directionalization of theexplosive energ-yin adownward direction to be obtained. Thus, assumethat a vertical co-planar explosive charge arrangement of the form shownin Fig. 4 of the drawings is employed using only three charges. With thewave length of'the undesired energy known from test data obtained in themanner explained bove, the outer shot holes maybe spaced aparta.distance equal to onehalf' the wave lengthof the undesired wave energy.When this spacing is used, maximum cancellation of undesiredwaveenergyis obtained in the manner previously explained when these twoouter charges are simultaneously detonated; With the center explosivechargel'lb located midway between the twoaoutercharges Via, the delaytime or factor DTnecessary to produce undesired wave cancellation asbetween thecenter charge lib and each of theoutercharges ilmmay bereadily determined from the-above equation and the brushes 33 and 34:0fthe-commutating device 25 correspondingly adjusted; to. produce. thisdelay. interval; At this'point, the necessary steps have; beengtaken:to. produGerma-Ximurn su pressionofundesired wave energy; propagation ina.

There; remains; only the a horizontal direction; step ofobtaining,maximum;directionalization in a downwarddirectionoftheexplosive energy developedlupon detonation 10f thecharges. This isaccomplished by appropriate selection of; the only remaining unfixedparameter, i. e., the depth of the center charge [lb relative to thedepth of the two outer charges Hag, This parameter may be determined onan experimental basis to produce maximum directionalization of theexplosive energy downward.

Although the desired, ends may. be accomplished in the mannerdescribedabove through the use of non-shaped explosive charges, they arematerially enhanced" by employing cavitated chargesof the characterillustrated in Fig. 3 of the drawings; As, there. shown, the explosive,charge ii; is, comprisedof; a; body; of explosive;

material, It, such, for example; as seismpgel A,

manufactured by E. I. duPont de Nemoursand:

Company, Inc., or the like, disposed within a closed container l9 andadapted to be detonated by means of a conventional'capil'embedded'in thetop portion thereof. usual electricalignition element adapted to beelectrically connected to the detonator l t by means :of a pair ofinsulated'conductors forming a cable Mb. Atthe bottom end'thereof, thebody of explosive'material. |8,is provided with a conical cavity 2iwhich may be-formed through the use of a metallic conical shaped linerhaving its peripheral edge joined to the walls of the container l9 at apointremovedfrom the lower end of this container. In order to enhancethe directional explosive eifect obtained through provision of theconical cavity in the lower end of the This cap includes the in Fig. 1,2 and 4 of the drawings.

explosive body I 8, the mouthof this-cavity should be separated from theadjacent bottom of theshot hole by a predetermined stand-off distance.This is conveniently accomplished by extending the sides of thecontainer [9 and by securing the optimum stand-off distance shouldbeap-- proximately one and one-half times the diameter of the body [8. Aswill be understood, if theenclosed explosive charges are to be used inshot holes containing loose or fluid material, at least the stand-onportion of the container shouldbe sealed. In addition, if the closedstand-off portion of the container is sealed and evacuated, a

greatly increased directional effect is obtained. The type of explosiveused is not critical although best results have been obtained with thoseex.-.

plosives which have the highest velocity ofdetonation and are reasonablyplastic. mentioned characteristic is of importance, since the contact ofthe explosive with the concavity liner, if used, must beuniform over theentire area of contact between the explosive material and the liner inorder to obtain best results.

As will be understood from the above explanation, cavitated charges I]of the specific form just described are disposed in the shot holesI5,and It with the cavities 2 I thereof facing downwardly.- Preferably,the prongs 22 of each charge are forced into the bottomsurface of theshot-hole, to insure maintenance of the charge in anup right position.After the charge is thus properlyplaced in the shot hole, the cableconductors Mb extending-to the top of the shot hole are wired to thedetonator in the manner illustrated in Fig.- 4 of-thedrawings. Wheneachcharge is detonated, the explosive energy is substantially all directedin a downward direction due to the efiect of the; concavity provided atthe lower endof the.

Hence a relatively small amountofseismic energyof the interfering typeis generated charge.

and, propagated laterally from the shot hole; Moreover, due tothe highdegree of directionalization of the generated energy in the desired-direction, only a small amountof explosive matem rial is required toproduce a large amountof seismic energy propagation in a downwarddirection. In fact, in certain areas where conditions of seismic wavepropagation are highly favorable, the-use of a single cavitated chargeI! of. the character shownin Fig. 3 of the drawing and utilizing only arelatively small amount of explosive material maybe found toproduceentirely acceptable seismic records without resorting to use ofthe multi-sh-ot systems illustrated upon conditions, the singlecavitated charge I! as used in such areas may be'disposedin shot.

holes drilled through oronly partially through the weathering layer 9.In any case, however, a markedsaving in the amount of explosive mate-.rial required is realized.

From the foregoing explanation it will be understood that the presentinvention represents-a marked improvement in the art of seismic wavegeneration in that it permits a much more efii- The last Depending.

cient use of explosives with an accompanying decrease in the amount ofinterfering energy produced. Also, material labor savings may berealized by virtue of the fact that shallower shot holes may be usedwithout afiecting the character of the seismic records obtained.

While different embodiments of the'invention have been described, itwill be understood that various modifications may be made thereon whichare within the true spirit and scope of the invention as defined in theappended claims.

We claim:

1. The method of generating seismic waves in the earth and of at leastpartially suppressing horizontal propagation of certain undesired wavesof a predetermined frequency, which comprises drilling a group of shotholes around the circumference of andat the center of a circle having aradius equal to substantially one half the wavelength of the waves ofsaid predetermined frequency, the depth of the center hole being lessthan the depths of the other holes, depositing explosive charges in saidholes, and simultaneously detonating said charges.

2. The method of generating seismic waves in the earth and of atleastpartially suppressing horizontal propagation of certain undesired wavesof a predetermined frequency, which comprises drilling a group of shotholes at spaced points around the periphery of a surface of revolutionand at a point along the axis of said surface of revolution, saidsurface of revolution having a radius equal to substantially one halfthe wave length of the waves of said predetermined frequency at thevelocity of propagation in the strata beneath the weathering layer ofthe earth, said holes all extending through the weathering layer and thedepth of the cent-er hole being less than the depths of the other holes,depositing explosive charges in said holes, and simultaneouslydetonating said charges.

3. The method of generating seismic waves in the earth and of at leastpartially suppressing horizontal propagation of certain undesired wavesof predetermined frequency which comprises drilling a group of shotholes around the periphery of a surface of revolution and at a pointalong the axis of said surface of revolution having a radius equal tosubstantially one half the wave length of the waves of saidpredetermined frewhich comprises detonating an explosive charge at agiven depth in the earths crust, and simultaneously detonating aplurality of additional explosive charges at spaced points surroundingthe point of detonation of said first-mentioned charge and at depthsgreater than the depth of said first-mentioned charge.

6. The method of producing directional propagation of seismic wavesthrough the earth,

quency at the velocity of propagation in the weathering layer of theearths crust, said holes all terminating in the weathering layer and thedepth of the center hole less than the depths of the other holes,depositing explosive charges in said holes, and simultaneouslydetonating said charges.

4. The method of producing directional propagation of seismic wavesthrough the earth and at least partially suppressing horizontalpropagation of certain undesired waves of a predetermined frequency,which comprises detonating a plurality of explosive charges atsubstantially the same depth in the earths crust and at points spacedaround a circle having a radius equal to substantially one-half the wavelength of the waves of said predetermined frequency, and simultaneouslydetonating another explosive charge substantially at the center of saidcircle and at a depth less than the depth of the other explosivecharges.

5. The method of producing directional propagation of seismic wavesthrough the earth,

which comprises detonating a cavitated explosive charge having itscavity directed downward at a given depth in the earths crust, andsimultaneously detonating a plurality of additional cavitated explosivecharges having their cavities directed downwardly at spaced pointssurrounding the point of detonation of said first-mentioned charge andat depths greater than the depth'ofysaid first-mentioned charge.

'7. The methodof generating seismic waves in the earth, which comprisespositioning three charges in spaced relation with on of said chargesdisposed between the other two charges, detonating said charges topropagate explosive forces downward from the three charge locationpoints to a common meeting zone disposed below the charge locationpoints, thereby to produce resultant explosive forces, and delaying thearrival of the explosive forces from the location point of said onecharge at said common meeting zone to directionalize said resultantexplosive forces downward and away from the location point of said onecharge and said common meeting zone.

8. The method of generating seismic waves in the earth, which comprisespositionin three charges in spaced relationship with One of said chargesdisposed between and displaced above the line connecting the other twocharges, and simultaneously detonating all of said charges to produceexplosive forces which are propagated to a common meeting zone disposedbelow and substantially in vertical alignment with said one charge andwhere said explosive forces interact to produce resultant forcesdirectionali ed downward from said common meeting zone and the locationof said one charge.

JAMES E. HAWKINS. WILLIAM E. PUGH.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Num er Name Date 1,867,098 Rieber July 12, 19321,899,970 McCollum Mar. '7, 1933 2,064,451 Voorhees Dec. 15, 19362154,548 Weatherby Apr. 18, 1939 2,203,140 Green June 4, 1940 2,331,080Petty Oct. 5, 194 2,340,314 Farnham Feb. 1, 1944 2,399,211 Davis Apr.30, 1946 FOREIGN PATENTS Number Country Date 28,030 Great'Britain of1911 701,747 France Jan. 13, 1931 OTHER REFERENCES Popular Science,article on hollow charges. February 1945. Pages 66 and 6'7. (Copy inDiv. 30 102/56.)

